The present invention relates generally to the field of offshore oil and gas drilling, and more particularly to a method of and system for drilling offshore oil and gas wells in which buoyant substantially incompressible articles are injected into the drilling fluid column at one or more injection points to reduce the density of drilling fluid column above the injection point or points, thereby to adjust or alter the drilling fluid pressure gradient over selected portions of the drilling fluid column.
With conventional offshore drilling, a riser extends from the sea floor to a drill ship. As is well known in the art, drilling fluid is circulated down the drill stem and up the borehole annulus, the casing set in the borehole, and the riser, back to the drill ship.
The drilling fluid performs several functions, including well control. The weight or density of the drilling fluid is selected so as to maintain well bore annulus pressure above formation pore pressure, so that the well does not xe2x80x9ckickxe2x80x9d, and below fracture pressure, so that the fluid does not hydraulically fracture the formation and cause lost circulation. In deep water, the pore pressure and fracture pressure gradients are typically close together. In order to avoid lost circulation or a kick, it is necessary to maintain the drilling fluid pressure between the pore pressure gradient and the fracture pressure gradient.
With conventional riser drilling, the drilling fluid hydrostatic pressure gradient is a straight line extending from the surface. This hydrostatic pressure gradient line traverses across the pore pressure gradient and fracture pressure gradient over a short vertical distance, which results in having to set numerous casing strings. The setting of casing strings is expensive in terms of time and equipment.
Recently, there have been proposed systems for decoupling the hydrostatic head of the drilling fluid in the riser from the effective and useful hydrostatic head in the well bore. Such systems are referred to as dual gradient drilling systems. In dual gradient systems, the hydrostatic pressure in the annulus at the mud line is equal to the pressure due to the depth of the seawater and the pressure on the borehole is equal to the drilling fluid hydrostatic pressure. The combination of the seawater gradient at the mud line and drilling fluid gradient in the well bore results in greater depth for each casing string and a reduction of the total number of casing strings required to achieve any particular bore hole depth.
There have been suggested three mechanisms to achieve dual gradient system. One suggested mechanism is continuous dumping of drilling fluid returns at the sea floor. This suggested mechanism is neither environmentally practical nor economically viable.
The second suggested mechanism is gas lift, which involves injecting a gas such as nitrogen into the riser. Gas lift offers some advantages in that it requires no major subsea mechanical equipment. However, there are some limitations associated with gas lift. Since gas is compressible, there are limitations on the depth at which it may be utilized and extensive surface equipment may be required. Additionally, because the gas expands as the drilling fluid reaches the surface, surface flow rates can be excessive.
The third suggested mechanism to create a dual gradient system is pumping the drilling fluid from the underwater wellhead back to the surface. Several pumping systems have been suggested, including jet style pumps, positive displacement pumps, and centrifugal pumps. Sea floor pump systems provide the flexibility needed to handle drilling situations, but they have the disadvantage of high cost and reliability problems associated with keeping complex pumping systems operating reliably on the sea floor.
The present invention provides a multi-gradient method of and system for drilling a well bore. Briefly stated, the system of the present invention injects buoyant substantially incompressible articles at one or more injection points into the column of drilling fluid associated with the well bore. An injection point may be positioned in a marine riser connected between a subsea wellhead and a surface drilling location, a cased section of the well bore, or an open hole section of the well bore. Preferably, the substantially incompressible articles comprises hollow substantially spherical bodies.
In one embodiment, a conduit is connected between the surface location and an injection point in the riser. A slurry containing the substantially incompressible articles is injected into the conduit at the surface location. In one embodiment, the slurry comprises a mixture of the substantially incompressible articles and drilling fluid. The drilling fluid may be of the same weight and composition as the primary drilling fluid being circulated in the well bore, or it may be of a lesser weight. The drilling fluid and incompressible article slurry may be injected directly into the riser. Alternatively, the incompressible articles may be separated from the drilling fluid prior to injection, thereby to increase the concentration of incompressible articles injected to into the riser. The separated drilling fluid is returned to the surface.
The slurry may alternatively comprise a mixture of the substantially incompressible articles and water. In the water slurry embodiment, the means for injecting the substantially incompressible articles includes means for separating the substantially incompressible articles from the water prior to injecting the substantially incompressible articles into the riser. In one embodiment, the means for separating the substantially incompressible articles includes a vessel positioned adjacent the injection point. The vessel is gas-pressurized to form a water-gas interface. A slurry inlet is positioned in the vessel below the water-gas interface and coupled to the conduit. A water outlet is positioned in the vessel below the water-gas interface. An article outlet positioned in the vessel above the water-gas interface and coupled to the injection point.
The system of the present invention may include means for recovering the incompressible articles from the drilling fluid returned to the surface location from the riser. In one embodiment, the means for separating the incompressible articles from the drilling fluid includes a screen device for separating the incompressible articles and drill cuttings from the drilling fluid. The screen device has a mesh size and the incompressible articles are larger than the mesh size. The system of the present invention further includes means for separating the incompressible articles from the drill cuttings. The means for separating the incompressible articles from the drill cuttings may include a water-filled vessel positioned to receive the incompressible articles and the drill cuttings from the screen device. The drill cuttings sink and the substantially incompressible articles float, thereby allowing the substantially incompressible articles to be recovered from the surface of the water in the vessel.
In an alternative embodiment, the incompressible articles are mixed with the primary drilling fluid. The mud pumps pump the mixture of incompressible articles and primary drilling fluid down the drill string to an internal injection point defined by a drill string separation and injection device positioned in the drill string near the depth of the seabed. The drill string separation and injection device separates the incompressible articles from the drilling fluid and injects the separated articles into the riser. The separated drilling fluid continues down the drill string to the bit and back up the annulus to the riser, where it mixes with the with the incompressible articles for return to the surface. The drill string injection method does not require that the incompressible articles be separated from the drilling fluid returned to the surface.
Preferably, the substantially incompressible articles are injected into the drilling fluid column at a rate sufficient to reduce the density of drilling fluid above the injection point to a predetermined density. The density p of the drilling fluid in the column is determined according to the equation   p  =                              (                      100            -            v                    )                ⁢                  p          f                    +              vp        s              100  
where
pf is drilling fluid density without the substantially incompressible articles;
ps is the density of the substantially incompressible articles; and
v is the concentration of the substantially incompressible articles. In the drilling fluid slurry embodiment of the present invention, the density p of drilling fluid in the riser is determined according to the equation   p  =                              p          m                ⁢                  Q          m                    +                        p          s                ⁢                  Q          s                                    Q        m            +              Q        s            
Where
pm is the drilling fluid density without the substantially incompressible articles;
ps is the density of the slurry;
Qm is the drilling fluid flow rate; and,
Qs is the slurry flow rate.